1. Field of the Invention
The present invention relates to driving LED (Light-Emitting Diode) lamps and, more specifically, to dimming the LED lamps.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
The use of LEDs in lighting applications is expected to expand, as they provide significant advantages over incandescent lamps (light bulbs) in power efficiency (lumens per watt) and spectral quality. Furthermore, LED lamps represent lower environmental impact compared to fluorescent lighting systems (fluorescent ballast combined with fluorescent lamp) that may cause mercury contamination as a result of fluorescent lamp disposal.
However, conventional LED lamps cannot be direct replacements of incandescent lamps and dimmable fluorescent systems without modifications to current wiring and component infrastructure that have been built around incandescent light bulbs. This is because conventional incandescent lamps are voltage driven devices, while LEDs are current driven devices, requiring different techniques for controlling the intensity of their respective light outputs.
FIG. 1 illustrates the operating characteristics of conventional incandescent lamps. As shown in FIG. 1, the amount of light produced in an incandescent lamp is proportional to the 3.4th power of the ratio of the Root-Mean-Square (RMS) input voltage, which is the square root of the mean value of the square of the instantaneous input voltage to the incandescent lamp. Thus, if the RMS voltage is reduced by one half (½), the light output is reduced to (½)324 or about one-tenth ( 1/10). In short, the amount of light produced is positively proportional to the RMS input voltage.
FIG. 2 illustrates a typical dimmer wiring configuration in conventional residential and commercial lighting applications. Predominantly, incandescent lamps operate off of alternating current (AC) systems. Specifically, a dimmer switch 10 is placed in series with an input voltage source 15 and the incandescent lamp 20. The dimmer switch 10 receives a dimming input signal 25, which sets the desired light output intensity of incandescent lamp 20. Control of light intensity of the incandescent lamp 20 is achieved by adjusting the RMS voltage value of the lamp input voltage (V-RMS) 30 that is applied to incandescent lamp 20. Dimming input signal 25 can either be provided manually (via a knob or slider switch) or via an automated lighting control system.
Many dimmer switches adjust the V-RMS by controlling the phase angle of the AC-input power that is applied to the incandescent lamp to dim the incandescent lamp. Phase angle control can be provided by switching devices such as a TRIAC device. FIGS. 3A, 3B, and 3C illustrate typical lamp input voltage waveforms which are output by dimming switches that employ phase angle switching. FIG. 3A illustrates lamp input voltage 30 when the dimmer switch 10 is set to maximum light intensity, as the voltage signal from the input voltage source 15 is unaffected by the dimmer switch 10. FIG. 3b illustrates lamp input voltage 30 with a slight dimming effect as the dimmer switch 10 eliminates partial sections 32 of the input voltage source 15. FIG. 3C illustrates lamp input voltage 30 with an increased dimming effect by further increasing the portions 34 of the input voltage source 15 that is switched by dimmer switch 10. Controlling the phase angle is a very effective and simple way to adjust the RMS-voltage supplied to the incandescent bulb and provide dimming capabilities.
However, conventional dimmer switches that control the phase angle of the input voltage are not compatible with conventional LED lamps, since LEDs, and thus LED lamps, are current driven devices. Light output from the LED lamp depends on the current through the LEDs. FIG. 4 illustrates the operating characteristics of a conventional LED. As shown in FIG. 4, the luminous flux (in lumens) from the LED increases as the LED's forward current (in mA) increases.
FIG. 5 illustrates a conventional LED lamp installed in a conventional dimmer wiring configuration in residential and commercial applications, similar to that illustrated in FIG. 2. Typically, LED lamps that are connected directly to AC power contain a current regulation control circuit to control the forward current through the LEDs. Referring to FIG. 5, LED lamp 50 is connected in series to AC input voltage source 15 through dimmer switch 10, and includes LED device(s) 51 and LED driver(s) 52. LED driver 52 monitors and regulates the forward current 53 of LED 51 and provides a constant current source over an input voltage range. Even if dimmer switch 10 adjusts the lamp input voltage 57 to the LED lamp 50 in response to dimming input signal 25, LED driver 52 maintains a fixed forward current 53 through LED 51. Thus, conventional LED lamps 50 may have only a fixed light intensity output from the LED lamp 50 over a variable range of the lamp input voltage 57. Thus, conventional LED lamps 50 may not be dimmed using conventional dimmer switches 10 as shown in FIG. 5, commonly found in typical residential and commercial settings.